A typical form of a metering device for a water-activatable agent is a WC sanitising device which is operative to provide a metered feed of the water-activatable agent into the flushing water, each time that the WC unit is flushed. Such a device may comprise a container which accommodates a carrier body for the water-activatable agent. The container is provided with openings in a mesh or lattice-like configuration so that the carrier body for the water-activatable agent which may be for example a water-activatable perfume or like material, can be replaceably disposed in the container. The carrier body for the water-activatable agent will usually comprise a water-soluble granulate material so that after a number of flushing operations the carrier body is itself consumed. That form of metering device suffers from the deficiency that it is only precisely during a flushing operation that the water-activatable agent is fed into the flushing water, by virtue of partial dissolution of the carrier body, and thus disappears into the drainage system simultaneously with the flushing water. As a result, the actually desired effect of the water-activatable agent, after a flushing operation, is of only a minimal extent with that kind of device, being of the order of magnitude of about 10% whereas about 90% of the water-activatable agent simply disappears into the drainage system without actually producing its proper or full effect.